The invention relates to a device and a method in the production of a fiber web and particularly relates to a device in the production of a fiber web for drying the fiber web by utilizing a moving metal belt.
An equivalent type of prior art is represented by patent specification WO 03/064761 A1 which describes a calender based on a metal belt. This specification can be considered the closest description of prior art.
Furthermore, a prior-art yankee cylinder is depicted in specification U.S. Pat. No. 6,154,981 which describes intensifying the heating of a hood covering the yankee cylinder.
In a tissue machine, a fiber web is dried on the shell of a large cylinder, i.e. the yankee cylinder. In addition to operating as the transfer path of heat for drying the fiber web, the yankee cylinder has three other functions: conveying the fiber web during drying, operating as a roll during hot pressing and operating as a base in the creping process.
Commonly, the drying of the fiber web takes place through several repeated stages which include evaporation, removal of evaporated steam from the surface of the dryer, condensation, and capillary drifting of water onto the evaporation surface. This process occurs in the same way on that outer surface which is against the hood and in which bi-directional flow takes place through evaporation and drifting. The process continues and becomes stable until the sole remaining water is bound in the fibers. The final drying is slower and requires more energy.
The creping of the fiber web is a precisely manageable operation which requires a controlled balance between the adhesion of the fiber web on the cylinder surface, the physical properties of an uncreped fiber web and forces applying to a doctor blade. The creping mainly takes place by means of the doctor blade from the surface of the yankee cylinder using suitable creping geometry in the settings of the doctor blade. The fiber web folds into slightly cross-directional creases and its machine-directional length shortens for 10-25 percentages. The machine-directional contraction is considered by setting the speed of the winder equivalently to a lower speed. The creping increases the bulk of the fiber web and improves softness, absorbency and stretch. The creping decreases tensile strength. In the case of tissue, its final quality and particularly its softness are greatly dependent on creping.
In dry-creping tissue machines, the fiber web is creped on the dryer when the dry content is 93-98 per cent. Machines in which the creping occurs in the dry content of less than about 90 per cent are called wet-creping machines. These machines further include a second drying unit.
The drying capacity of the tissue machine is often limited by the size of the yankee cylinder. The speeds of the tissue machines have increased along with twin-wire formers. The higher drying capacity has led to the use of larger yankee cylinders. Now, the diameters of yankee cylinders in tissue machines of the highest speeds are in the range of 5,500 mm.